Polymers are widely used in container, sealing, and protective applications. Elastomeric polymers, for example, are used as containers and also for seals such as the familiar O-ring seal. In the container application, the polymeric material is used as a container or as the lining in a container of another material. In the sealing application, the polymer seal is positioned at a joint between two components and the components are tightened together with the polymer seal between them. The polymer seal reduces the leakage of fluids (i.e., either gases or liquids) through the joint. In each application, it is important that the polymeric material minimize the permeation of diffusing species of interest therethrough.
In most applications, the conventional, commercially available polymeric materials and “hermetic” packaging are fully satisfactory to prevent the macroscopic leakage of fluids. Other applications have more stringent requirements, and even a minute leakage of tiny amounts of fluid through the container wall or past the seal cannot be tolerated. In one example, electronic circuitry such as a guidance system may be damaged by water vapor that permeates into its container, condenses, and corrodes or short circuits the electronics. In another example, water vapor leaking into an optical system may fog the lenses, mirrors, detectors, and other optical components. In these applications, leakage of tiny amounts of fluids into the containers or past the seals can have disastrous effects, particularly where the pre-service storage period may be long, the service life is long, or the environments are demanding. Conventional “hermetic” packaging and interior pressures of inert gases are not sufficient to reduce the leakage sufficiently for some of these applications, and damaging amounts of water vapor or other contaminant fluids may reach the sensitive components.
The mechanical performance of the seals in such stringent applications has been investigated. That is, structures, sizes, thicknesses, loads, and other parameters for containers, and groove sizes and profiles for seals have been studied and optimized. This work has not led to the required container or sealing performance for the most demanding applications.
Instead, it has become apparent that the permeation of the fluid through the polymeric material is the most significant cause of fluid intrusion. With this fact in mind, various choices of polymeric materials have been considered. For example, butyl rubber elastomeric materials are found to have very low moisture permeability, but they have less-than-optimal chemical stability in many common environments such as those containing jet fuel. Nitrite rubber, on the other hand, is highly stable in an environment containing jet fuel, but has a limited lifetime in air. Fluorocarbon and ethylene propylene elastomeric materials have good chemical stability, but higher water permeation. Silicone and fluorosilicone materials have excellent chemical stability, but permit permeation at a rate 300 times greater than that of butyl rubber.
No elastomeric container or seal material has been found with a combination of good chemical stability and sufficiently low permeability for the most demanding applications. As a result, a material with a higher permeability than desired is often chosen for an application because of other considerations such as physical properties, chemical stability and compatibility, and/or thermal properties.
There is a need for improved polymeric materials with acceptable mechanical and chemical properties, and reduced permeability, for use in containers, seals, and other applications. The present invention fulfills this need, and further provides related advantages.